1. Field of the Invention
The present invention relates to a method to achieve higher light-emission efficiency and longer operating life for a top-emission-type organic EL device, in which light is taken out from the upper electrode of the device.
2. Description of the Related Art
A display apparatus in which an organic EL device can be practically used as a display device is currently under active research and development.
Particularly, a top-emission-type organic EL device, in which light is emitted through an upper electrode provided on the opposite side of a substrate, has an advantage over a bottom-emission-type organic EL device, in which light is emitted through a lower electrode on the side of a substrate, especially when used in an active-matrix-type display apparatus wherein TFTs (thin film transistors) are employed. This is because the aperture ratio of the display device can be increased, since there is no light shielding by the TFT matrix, and higher brightness and longer life can be achieved.
FIG. 3 is a sectional side view of relevant parts illustrating a bottom-emission-type organic EL device of the conventional art, wherein 1 indicates a transparent substrate, 2 indicates an anode comprising a transparent electrode, 3 indicates an organic layer comprising a light-emitting layer (not shown), and 4 indicates a cathode.
In the bottom-emission-type organic EL device shown in FIG. 3, light emitted from the light-emitting layer in the organic layer 3 is taken out from the transparent substrate 1 via the anode 2 (see, for example, L. S. Hung, C. W. Tang and M. G. Mason, Applied Physics Letters, Vol. 70 (2), 152 (1997)).
This bottom-emission-type organic EL device generally uses a glass for the transparent substrate 1, an ITO (indium tin oxide) for the anode 2 comprising a transparent electrode, and an Al for the cathode 4. Further, there are cases of using a LiF/Al structure, which is an Al cathode having an LiF electron-injection-layer on its interfacing surface to the organic layer 3 to facilitate the electron-injection from the cathode 4 to the organic layer 3.
FIG. 4 is a sectional side view of relevant parts illustrating a top-emission-type organic EL device of the conventional art, wherein 11 indicates a nonconductive substrate, 12 indicates an anode, 13 indicates an organic layer comprising a light-emitting layer (not shown), and 14 indicates a cathode comprising a light-transmitting electrode.
In the top-emission-type organic EL device shown in FIG. 4, light emitted from the light-emitting layer in the organic layer 13 is taken out from the cathode 14 comprising a light-transmitting electrode.
A transparent electrode, e.g. an ITO, is typically used for the cathode 14 in the top-emission-type organic EL device. However, it is said that when a semi-transparent electrode, e.g. an Mg thin film or an Ag thin film, is used for the cathode 14, and a light-reflective electrode, e.g. a Pt, Au or a Cr thin film, is used for the anode 12, a multiple interference occurs between the cathode 14 and the anode 12, creating a microcavity effect, whereby an emission spectrum is made steeper than that of the bottom-emission-type organic EL device, hence the color purity is increased (see WO01-039554, for example).
Further improvement in light-emission-efficiency can be expected by using a highly light-reflective metal such as an Ag or an Al for the anode 12. However, the electron hole may not be easily injected directly from the Ag or Al layer into the organic layer 13, causing a rise in operating voltage of the organic EL device.
To overcome such a problem, methods to improve the hole injection property of the top-emission-type organic EL device, e.g. designing an anode to have an Al—Cu/Ni/NiOx/V2O5 structure (see the specification of Japanese Patent No. 3488474, for example), an Ag/ITO structure (see the Japanese Patent Application Laid-Open No. 2004-192890, for example) and an Al/Ni structure (see J. Appl. Phys., Vol. 94, No. 8, 5290 (2003), for example), have been known.
In addition, as a method to achieve not only better color purity but also better light-emission efficiency compared to a conventional bottom-emission-type organic EL device, there has been disclosed use of a top-emission-type organic EL device in which an Ag anode, a hole-injection layer comprising a CFx and a MoOx and an Ag semi-transparent electrode in the cathode are used, and the thickness of an organic layer is optimized (see Japanese Patent Laid-Open Publication No. 2004-228082, for example).
As a technique using an Ag thin film as the cathode, there has been reported a method wherein an Al super-thin film having a thickness of 0.6 nm is sandwiched between a LiF electron-injection layer and an Ag thin-film cathode so that good voltage-current density property is obtained (see Japanese Patent Application Laid-Open No. 2001-52878, for example).
Therefore, it is said that the top-emission-type organic EL device, utilizing the above-mentioned microcavity effect, has the potential to realize an active-matrix-type display apparatus having advanced color purity and light-emission efficiency, high degree of brightness, light-emission efficiency and color reproducibility.
However, according to the present inventor's findings, the top-emission-type organic EL device still has technical problems associated with the operation life, and no case of realization of a lifetime long enough to endure practical use has been reported. In particular, a device comprising an Al/Ni anode disclosed in the above-mentioned J. Appl. Phys., Vol. 94, No. 8, 5290 (2003), for example, exhibits inferior life property.